This invention relates to solar cell arrangements and more particularly to the protection of solar cells against reverse current through them.
In one type of solar cell (or photovoltaic cell), a voltage is developed across a p-n junction in a semiconductor when it is irradiated by photons. A plurality of solar cells may be connected together in an array to provide a power supply, such arrangements commonly being used in spacecraft and satellites for example. During operation, there may be circumstances where a single solar cell is shadowed or is faulty, and therefore not producing power, whilst the remainder of a string of cells with which is connected in series is illuminated and producing power. This may lead to current being driven in reverse through the shadowed solar cell. Power is dissipated within the cell which is proportional to the reverse voltage across it. If the power density is sufficiently high, the cell may irreversibly break down.
One way in which this problem may be alleviated is to use a protection diode wired across one or several solar cells in reverse parallel with them. If one of the cells across which it is connected is driven into reverse bias, the protection diode diverts current from the cell. This limits the power dissipation in the shadowed cell by both limiting the voltage across the shadowed cell and by passing the reverse current.
The present invention seeks to provide an improved solar cell arrangement which is particularly advantageous when applied to GaAs solar cells and cascade solar cells, although other types of solar cell may also benefit from it.
According to the invention there is provided a solar cell arrangement comprising: a solar cell which in plan view has substantially mutually orthogonal first and second sides with a third side between them; and a protection diode electrically connected to the solar cell and located next to the solar cell in a region bounded by the third side and projections of the first and second sides.
Use of the invention enables each solar cell in an array to be protected individually. As the protection diode is located to the side of the solar cell, no special coverglass configuration is required and conventional automatic glassing and interconnecting equipment may be used. This is particularly attractive to users who acquire cells for later incorporation into an array as the array may be assembled without needing to give special consideration to protection of the cells or modifying the assembly process. Use of the invention also allows the solar cell arrangement to be mounted on a panel in a conventional manner as may be no more fragile than a cell which does not incorporate diode protection and the cell also has a substantially conventional profile.
By using the invention, it possible for a solar cell to have its own dedicated protection diode without appreciably increasing the overall area required to accommodate it, which is particularly advantageous where the cell is included in an array. The diode is located in a region which might otherwise be taken up by part of the solar cell itself. This may lead to a loss of active solar cell area. However, in some cases, the diode may be included without any loss in the area occupied by the solar cell and still take advantage of the invention. For example, where a solar cell is manufactured from a circular wafer forming the substrate on which the p-n junction is fabricated, the cell dimensions are sometimes sufficiently large in comparison that a regular, rectangular solar cell cannot be derived from the wafer without two of the corners being lost. In this case, the protection diode may advantageously be located in one of the cropped corners of the otherwise rectangular solar cell.
The diode is conveniently of triangular cross section in plan view where the solar cell has a straight third side extending between first and second sides so as to fit in the triangular region so defined. However, the diode could be other shapes, for example, of circular or square cross section. It is preferred that the diode is wholly located in the region bounded by the third side and projections of the first and second sides. This facilitates integration of the solar cell arrangement into an array where it is mounted closely spaced from adjacent arrangements and interconnected therewith. However, the diode could be slightly larger than the region and still offer advantages over conventional arrangements.
In a preferred embodiment of the invention, the third side joins the first and second sides and is oblique to them. As mentioned above, this configuration of cell sometimes occurs in any case in conventional cells because of the processing techniques used. However, in another embodiment, the third side comprises two orthogonal straight edges to define a square notch at a corner of the solar cell. This is more difficult to cut in practice, however, and is thus not preferred.
The diode is preferably a silicon diode. The characteristics of such a diode can be particularly well controlled and made compatible with the requirements of the solar cell. In another embodiment, the diode is of the same material as the solar cell active material. For example, it may be an off-cut from the solar cell itself connected across the solar cell such that its p-n junction is in reverse to that of the solar cell.
The invention is particularly applicable to solar cell arrangements where the solar cell is a GaAs solar cell. GaAs cells are particularly vulnerable, tending to break down at lower reversible voltages than silicon solar cells, for example. Previously, some users of GaAs solar cells have pre-screened them prior to incorporating them in an array to eliminate those which are more likely to be break down in use. By using the invention, it is no longer necessary to carry out a pre-screening procedure saving time and expense, and also improving yield in manufacturing the cells. The invention is also applicable to cascade solar cells, also known as tandem solar cells. A cascade solar cell is one which includes two p-n junctions connected in series internally by a tunnel junction or some other means. The front p-n junction produces voltage in response to incident radiation of a relatively short wavelength and the rear p-n junction is tailored for longer wavelengths.
The solar cell in plan view may be rectangular or square except for a corner or corners which are cropped and in one of which the protection diode is located. The protection diode is in one embodiment mounted on the solar cell. This may be achieved, for example, by using a non-conductive adhesive to fix it to the side of the solar cell or by applying a bead of epoxy to join the diode to the back contact of the cell. In an alternative embodiment, which is more appropriate where thermal cycling during use is envisaged, the protection diode is spaced from the solar cell. The solar cell and protection diode may be electrically and mechanically connected by an interconnect or interconnects which are configured to allow for differential thermal expansion. The normally provided coverglass located on the front surface of the solar cell may be extended to project over the diode also, although this is not essential.
According to a first feature of the invention, a solar cell array comprises a plurality of solar cell arrangements in accordance with the invention. The invention enables the solar cell arrangements to be located close to one another, maximizing the area available for reception of incident radiation and generation of power.
According to a second feature of the invention, a solar cell array comprises a plurality of solar cell arrangements, each arrangement comprising a solar cell which in plan view has substantially mutually orthogonal first and second sides with a third side between them; and a protection diode located next to the solar cell in a region bounded by the third side and projections of the first and second sides and electrically connected to an adjacent solar cell.
By employing this second feature, the advantage of providing a dedicated protection diode for each solar cell in an array is maintained as is the compact arrangement which results from use of the invention. The protection diode may be fixed mechanically to the solar cell prior to incorporation into the array and the electrical connections to the adjacent solar cell made following mounting the solar cell arrangements on a panel.